This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-079243, filed in Mar. 20, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention generally relates to an optical element, and more particularly to an optical switch.
2. Description of the Related Art
Owing to today""s spread in optical communication technology, the amount of information transmitted through optical communication systems are growingly increasing. In response to the growing increase, today""s optical communication system transmits optical signals at numerous wavelengths through a single optical fiber by using a multiplex wavelength technique.
Meanwhile, an optical switching device serving to switch optical signals in a backbone communication network is required for the foregoing optical communication systems, in which the core for such optical switching device is considered to be an optical switch capable of switching the path of the optical signals at a high speed.
The polarization characteristic of the optical signals transmitted through the optical fiber is typically random, and therefore, the polarization plane thereof, normally, vibrates randomly.
Accordingly, even when an incoming optical signal has random polarization planes, the optical switch for the optical communication system using optical fiber is required to perform in a manner independent from such polarization planes.
Japanese laid-open publication No.4-234020 shows a waveguide type optical switch which separates TE mode and TM mode, and then, recombines the modes after switching the modes.
Japanese laid-open publication No.3-216622 shows an optical switch using an electro-optical effect which suitably selects the direction of an electro-optical crystal, to thereby achieve polarization independence with respect to the electro-optical effect.
FIG. 1 shows a structure of an optical switch according to the aforementioned Japanese laid-open publication No.4-234020.
With reference to FIG. 1, an optical switch formed on a GaAs substrate 1 on plane (111) has ridges 2 corresponding to plural separating and intersecting optical waveguides formed on the GaAs substrate 1. The ridges 2 have electrodes 3 disposed at separating portions and intersecting portions thereof.
In FIG. 1, an optical waveguide layer 1A is formed above the GaAs substrate 1 in a manner sandwiched between a conductive clad layer 1B and a non-doped clad layer 1C. The optical waveguides are formed as non-conductive type clad layers disposed on the non-doped clad layer 1C. Accordingly, an inputted optical beam is guided through the optical waveguide layer 1A along the ridges 2 by a refractive index effect of a non-conductive type clad layer of the ridges 2.
Thus structured, a change in refractive index due to an electro-optical effect in the optical waveguide layer 1A is induced by applying a controlling voltage between the electrodes 3 and an electrode disposed on a back surface of the substrate 1. As a result, the path of the optical signal guided along the ridges 2 can be switched.
FIGS. 2A and 2B show a structure of an optical switch according to the aforementioned Japanese laid-open publication No.3-216622.
With reference to FIGS. 2A and 2B, an optical switch formed on a GaAs substrate 5 includes an optical waveguide layer 5A sandwiched between a bottom side clad layer 5B and a top side clad layer 5C, which are respectively formed above the GaAs substrate 5 and doped as a reverse conductive type. Ridges 5D corresponding to an optical waveguide path are formed on the top side clad layer 5C. The ridges 5D have electrodes 6 formed thereon, and a back surface of the substrate 5 has an electrode 7 formed thereto.
Accordingly, the structure shown in FIGS. 2A and 2B also allows a change in refractive index due to an electro-optical effect in the optical waveguide layer 5A to be induced by applying a controlling voltage between the electrodes 6 and the electrode 7.
As the GaAs substrate, the structure shown in FIGS. 2A and 2B employs a GaAs single crystal being cut out in a manner extending in a (110) direction while having a main plane thereof tilting xcfx86 degrees from a (001) direction.
However, the foregoing conventional optical switches of optical guide types inevitably require to be formed having an extremely complicated structure for switching optical signals to multi-channels. Therefore, manufacture cost for the optical switches will correspondingly increase.
In the structure shown in Japanese patent laid-open publication No.4-234020, a TE mode and a TM mode are separated and are then recombined after optical switching is performed, so that polarization dependency (PMD: polarization mode dispersion) can be restrained. Such structure, however, has difficulty in preventing polarization dependency due to birefringence remaining in an optical system thereof since a GaAs crystal disposed on the GaAs substrate is employed as the optical system.
Meanwhile, although the structure shown in Japanese patent laid-open publication No.3-216622 is able to prevent the problem of polarization dependency, such structure requires the GaAs substrate to be cut out with a highly specific angle, and would therefore increase the cost for manufacturing the optical switch. Furthermore, the optical switch would require to be formed having an extremely complicated structure in order to achieve optical cross-connection of multiple channels.
Accordingly, the general object of the present invention is to solve the aforementioned problems by providing a novel and advantageous optical switch. A more concrete object of the present invention is to provide an optical switch having a simple structure, being able to prevent the problem of polarization dependency, and being able to easily achieve optical cross-connection of multiple channels.
In solving the foregoing problems, the present invention provides an optical switch which includes a slab waveguide having a first end surface and a second end surface, a first deflection portion being fixed to the first end surface and having a plurality of deflection elements formed thereon, a second deflection portion being fixed to the second end surface and having a plurality of deflection elements formed thereon, a first waveguide portion being optically coupled to the first deflection portion and having a plurality of channel waveguides formed therein, and a second waveguide portion being optically coupled to the second deflection portion and having a plurality of channel waveguides formed therein, wherein the slab waveguide has a waveplate disposed therein and arranged at a position where the distance between the waveplate and the first end surface is substantially equal to the distance between the waveplate and the second end surface.
It is preferable for the waveplate to be a half waveplate by which a polarization plane of an optical signal propagated through the slab waveguide is rotated 90 degrees. It is preferable for the slab waveguide to have a groove formed therein and arranged at a position where the distance between the groove and the first end surface is substantially equal to the distance between the groove and the second end surface, wherein the waveplate is fixed inside the groove in a state where a transparent medium is filled into a gap formed between the waveplate and the groove. It is preferable for the slab waveguide to have a first side wall surface and a second side wall surface in which the waveplate continuously extends from the first side wall surface and the second side wall surface. It is preferable for the first deflection portion and the second deflection portion to be crystals providing an electro-optical effect, wherein the deflection elements are electrodes to which electric voltage is applied. It is preferable for the first waveguide portion and the second waveguide portion to include a plurality of two dimensional lens arrays optically combined correspondingly with each channel waveguide of the plurality of channel waveguides.
It is preferable for the slab waveguide to be formed of a bottom clad layer having a planar shape disposed on a substrate, a core layer having a planar shape disposed on the bottom clad layer, and a top clad layer having a planar shape disposed on the core layer.
It is preferable for the waveplate to be contained in the groove formed in the slab waveguide. It is preferable for the groove to be formed traversing across the slab waveguide, wherein the groove is formed with a depth which reaches to the substrate. It is preferable for the gap formed between the groove and the waveplate to have a size no greater than several tens of micrometers, wherein the gap is filled with an optical adhesive agent which is transparent.
Since the optical switch of the present invention switches the path of optical signals inside a slab waveguide, the optical switch is not required to be formed with a complicated structure, even for providing optical cross-connection of multiple channels. In such a case, the polarization plane of the optical signals propagated through the slab waveguide rotates 90 degrees by disposing a waveplate (preferably a xc2xd waveplate) at a center portion of the slab waveguide. As a result, the TE mode signal light and the TM mode signal light become switched at a former half portion and a latter half portion of the half waveplate, in which the time lag between TE mode signal light and the TM signal light created in the former half portion of the slab waveguide is countervailed at the latter half portion of the half waveplate, to thereby compensate the polarization dependency in the optical switch.
Furthermore, in the present invention, the polarization dispersion loss (PDL) during the propagation of the TE mode optical signal and the TM mode optical signal through the flat slab waveguide, that is, the loss difference between the optical component in a direction normal to the flat core layer of the slab waveguide and the optical component in a direction parallel to the flat core layer of the slab waveguide is compensated at the former half portion and the latter half portion of the slab waveguide since the polarization plane of the optical signals is rotated 90 degrees at the half waveguide. The effect is created in the slab waveguide but also in the electro-optical crystal, and the microlens array. Accordingly, the optical switch of the present invention can be formed with a simple structure, and thus achieve a desirable polarization independent characteristic.
It is a general object of the present invention to provide an optical switch that substantially obviates one or more of the problems caused by the limitations and disadvantages of the related art.
Features and advantages of the present invention will be set forth in the description which follows, and in part will become apparent from the description and the accompanying drawings, or may be learned by practice of the invention according to the teachings provided in the description. Objects as well as other features and advantages of the present invention will be realized and attained by an optical switch particularly pointed out in the specification in such full, clear, concise, and exact terms as to enable a person having ordinary skill in the art to practice the invention.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an optical switch, including a slab waveguide having a first end surface and a second end surface, a first deflection portion being fixed to the first end surface and having a plurality of deflection elements formed thereon, a second deflection portion being fixed to the second end surface and having a plurality of deflection elements formed thereon, a first waveguide portion being optically coupled to the first deflection portion and having a plurality of channel waveguides formed therein, and a second waveguide portion being optically coupled to the second deflection portion and having a plurality of channel waveguides formed therein, wherein the slab waveguide has a waveplate disposed therein and arranged at a position where the distance between the waveplate and the first end surface is substantially equal to the distance between the waveplate and the second end surface.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.